gurai is used after a noun: nan JIKAN gurai kakarimasuka. (How many hours will it take?) here, jikan is a noun and it means "hour"
nan kurai kakarimasuka. (how long will it take?) here, nan is an interjection, so we use kurai
same for koro and goro
(please, don't let this question available for further answering, if you find your best answer, choose it as so. ^^)